This invention is directed to compositions that generate polymer microparticles upon polymerization. More specifically, the compositions contain free radical polymerizable monomers, oligomers or polymers, that polymerize or crosslink in the presence of an organoborane amine complex, upon exposure to an amine-reactive compound, or by exposure to heat or radiation. The reaction is carried out in a medium containing at least one poor or non-solvent for the resulting polymer, in order to form microparticles. The invention further relates to a process for making the microparticles, and to a method of encapsulating an active ingredient in a polymeric matrix using the composition.
Polymeric microparticles have many uses in the area of encapsulation of active ingredients, such as pharmaceutical agents, agricultural chemicals, vitamins, catalysts, curing agents, reactive species for curable materials, adhesion promoters in adhesives, flame retardants in plastics, fragrances, oils, and lotions for cosmetics. For example, the shelf life of a one part, i.e., in one package, curable silicone elastomer, may be extended substantially, by encapsulating curing catalysts such as tin or platinum in a polymeric resin particle. Such a process of encapsulation is described in European Patent 339 803 (Jan. 15, 1992), in which a platinum group metal catalyst that is microencapsulated in a thermoplastic resin, is recovered by atomizing a polymer emulsion, in which the catalyst containing microcapsules are held, and then entraining the resulting finely divided droplets, in a stream of heated gas, to rapidly evaporate the liquid on the exterior of the microcapsules, to form a free flowing powder. However, this spray drying operation is quite costly, and is limited in the range of particle sizes that can be obtained.
Polymeric fine particles are also useful in their own right in a variety of applications including their use as spacer beads in liquid crystal displays, electronic inks in electrophoretic displays, modifiers for adhesives, toners and toner additives for xerography and printing devices, additives for paints, pigments, powder coatings, thermoplastic and thermosetting materials, and packings for chromatographic columns.
Methods for making such polymer microparticles are known in the art. Mechanical grinding or crushing of bulk polymers may be used to form fine particles. Mechanical processes such as high speed rotary milling, or jet milling, however, have the inherent drawback of being post processing steps for the polymer. Additionally, such processing equipment is energy intensive, as well as being costly to implement and maintain. Therefore, the direct polymerization of microparticles is often preferred when possible.
These polymerizations are typically carried out as heterogeneous reactions such as emulsion polymerization, non-aqueous dispersion polymerization, precipitation polymerization, and suspension polymerization. A detailed review of such techniques is contained in an article by R. Arshady in the Journal of Colloid and Polymer Science, Volume 270, Pages 717-732 (1992).
While emulsion polymerization can result in fine uniform polymer microparticles, this technique suffers from the disadvantage that it requires surfactants that are often environmentally undesirable. Non-aqueous dispersion polymerization and precipitation polymerization utilize organic solvents, and therefore require removal of large quantities of organic solvents. While suspension polymerization does not require surfactants since the monomer is mechanically dispersed or suspended in water, the resulting particle sizes tend to be larger than what is afforded by the other techniques.
Recent microparticle polymerization technologies utilizing supercritical carbon dioxide as the reaction medium, can eliminate the need for handling large quantities of organic solvents, and are described by J. M. DeSimone et al., Science, New Series, Volume 265, No. 5170 Pages 356-359, (Jul. 15, 1994). However, these techniques require pressurized reaction equipment. Free radical polymerization to form microparticles by any of the previously mentioned heterogeneous polymerization techniques is accomplished through conventional peroxide or azo-type free radical initiators typically requiring elevated temperatures to initiate polymerization. While improvements have been made for each of these techniques, they still suffer from limitations in the process complexity.
For example, U.S. Pat. No. 5,852,140 (Dec. 22, 1998) describes a multi-step process for preparing a suspension of polymer particles, with an average diameter of from about 0.1-10 micron (μm). The first step of this process effects a bulk polymerization of a mixture of at least one monomer, a free radical polymerization inhibitor, and a stable free radical agent, until about 10-50 weight percent of the monomer has been polymerized. The second step is the dispersing of the bulk polymerization product with a high shear mixer into water containing a stabilizing component, to obtain a suspension of droplets having an average particle diameter of 0.1-10 μm, and then polymerizing the resulting suspension in water. Because the initiator is stable, both the bulk and suspension stages of the polymerization are conducted at a temperature of about 60-160° C., optimally 120-140° C. However, nothing in the public domain describes a process that uses organoborane amine complexes as initiators in the heterogeneous polymerization of microparticles.
Organoborane amine complexes themselves are known. For example, organoborane amine complexes used for the bulk polymerization of acrylic monomers, are described in U.S. Pat. No. 3,275,611 (Sep. 27, 1966). Organoboron compounds such as trialkylboranes by themselves, however, are pyrophoric in the presence of oxygen, so preformed complexes between organoboron compounds and amine compounds are required to have the benefit of imparting improved stability to organoboron compounds such as the trialkylboranes.
Recent modifications on the structure of organoborane amine complexes are also described in U.S. Pat. No. 6,706,831 (Mar. 16, 2004), and their use in acrylate based adhesives. The combination of alkylborane amine complexes with amine reactive decomplexing agents to initiate the polymerization of acrylic adhesives at room temperature, is also described in the '831 patent. While such compositions offer the advantage of rapid cure and adhesion to low energy surfaces, the compositions polymerize to form continuous, monolithic structures such as adhesives, rather than discrete microparticles.
This invention differs distinctly from the prior art of alkylborane-amine complexes in that it requires the presence of a poor or non-solvent for the polymerized product, during the polymerization process. For example, while the presence of a poor or non-solvent is undesirable for the direct formation of monolithic objects such as adhesively bonded joints or rubber parts in compositions known in the art, it has been found herein that this results in the unexpected rapid formation of discrete polymeric particles having an average primary particle size of less than about 10 μm in diameter. Therefore, due to the aforementioned limitations of the processes currently used to make polymer particles, and the encapsulation processes based thereon, there is a need for a facile, low cost route to polymer particles, that can be used for the encapsulation of active ingredients. In this regard, the composition and process according to this invention provide a rapid, low temperature means to prepare and recover polymer particles.